[Aroid-l] An Opinion on CO2 Injection
- Subject: [Aroid-l] An Opinion on CO2 Injection
- From: Ted.Held@hstna.com
- Date: Thu, 30 Dec 2004 09:20:11 -0500
This is in reply to the recent suggestion of CO2 injection to boost aroid growth. This technique has been touted by some in the aquarium plant hobby as the perfect remedy for weak plants. There is an impulse here from the manufacturers of these systems, which are fairly expensive, and the hobby literature contains many anecdotes endorsing the idea. I have never installed one in my setup, so I cannot attest to the efficacy of such equipment.
But since I do specialize in aquatic plants, I have been interested in these reports. The problem I have is that CO2 is not actually very soluble in water. A few parts per million is a lot for water at the typical pH 5 to 8 that is recommended for water plants. The lower the pH, the less soluble CO2 will be. At pH 5, the solubility of CO2 will be essentially nil. What happens, therefore, is that the excess gas "spills" to the atmosphere as bubbles. There are those in the hobby that interpret the Pepsi Cola-like effervescence and numerous attached subsurface bubbles as the huge amount of oxygen generated by all the enhanced photosynthesis taking place. This is humbug. The bubbles are just wasted CO2.
Since I grow many of my water plants "emersed" (i.e. the roots are submerged, but the leaves extend up into the air), I thought I could test this elevated CO2 idea by pumping the airspace above the plants with CO2. It turns out that this is a much more effective way of applying CO2 than in the dissolved form since one can basically create a 5% CO2 concentration in the atmosphere and the amount of exposure there is many times higher than what can be forced with dissolved CO2. My technique was to simply burn a candle in the closed space above the water. When the oxygen concentration goes from the normal 20% to 15% because of combustion, the candle is extinguished. I found that my oxygen level returned to approximately normal after 24 hours, at which point the candle would be lit anew. The short version of the story is that I observed no increase in plant growth or flowering in the 10 weeks that this regimen was in place. In fact, I e
nded the experiment when several of the plants exhibited evidence of reduced growth. I concluded that the effect of increased CO2 is not nearly so giddy as the injection people have maintained.
One possible explanation for the results reported by partisans of injection is that water (and air, for emersed plants) circulation is a definite help in the growth of water plants (as, for example, Cryptocoryne, an aroid genus of the Asian tropics). Perhaps the addition of injection resulted in better circulation where a particular grower previously had none. On the other hand, maybe the exuberant reports are simply hype. Maybe I will be attacked here for suggesting this dangerous, contrary opinion.
My larger point is to enquire to this list about any scientific studies on the effect of elevated CO2 (as in a greenhouse) on the growth of aroids. I am pretty sure that such studies have taken place for various food crops (tomatoes) and the effect has been investigated recently in connection with the Global Warming business. My understanding is that some plants will grow more rapidly with extra CO2 while others will not benefit. Does anyone know of any studies, one way or another, with CO2 and aroids?
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